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a7992 (Industrial) (OP)
7 Jan 09 15:42
We just found multiple pin hole leaks in Carbon Steel tubing in our HVAC unit.  The tubes carry 50# of steam (might have trace amounts of cyclohexlamine and hydrazine (.1-.2 ppm) at ~300 F.  Aluminum fins are touching (not actually attached) the steel and most (if not all) of the pinholes are located beneath the aluminum fins.  I'm concerned about galvanic corrosion but don't think it could be that since the Aluminum is the more active metal.  Could erosion-corrosion cause these pinholes?  Maybe it's just localized corrosion?  Finally, could galvanic corrosion occur at this location?  Any information would help!  Thanks!
SeasonLee (Mechanical)
7 Jan 09 20:44
Porosity, especially micro porosity could be the problem, suggest to impregnate and see the result.

Thanks

SeasonLee
 
unclesyd (Materials)
7 Jan 09 23:10
Could post a picture of the area around the leak?

Have you determine which side of the tube the leak originated from?

One reason for asking that I've seen Al fins wear small holes in thin wall CS tubes due to vibration of the fins.  
strider6 (Materials)
8 Jan 09 7:27
As unclesyd suggest the first thing is to define where, internal or external surfaces, the pinhole originate and also a metallurgical and chemical analysis.  One possibility is also internal corrosion.
I've found the article below interesting even if the materila of tubes is Cu and not Carbon Steel but it describes a type of corrosion similar to your case:

"In most failures of indoor coils, however, the
copper tubes will have been penetrated while there
will be little corrosion on the aluminum fins in direct
contact. Figure 2 shows pits in the copper tube
surface and the contacting aluminum fin surfaces. In
these cases the corrosion appears to be caused by a
direct attack of the corroding agent on the copper in
spite of the presence of the 'more sacrificial'
aluminum."

CORROSION OF ALUMINUM-FIN, COPPER-TUBE HEAT EXCHANGE COILS
https://txspace.tamu.edu/bitstream/handle/1969.1/4580/ESL-HH-02-05-30.pdf;jsessionid=27AD6D419B8F664E1BF0142324947512?sequence=1


hope this can help u

Regards

S.

Corrosion Prevention & Corrosion Control
 

a7992 (Industrial) (OP)
8 Jan 09 8:37
The holes look like they might be originating from the outside but I'm not exactly sure how to tell.  I do see some internal corrosion (probably erosion-corrosion or uniform corrosion) on the side that the holes are found on.  I'm confused because the holes look like they are originating from the outside but the inside is corroding.  Could the walls be getting thin enough in certain areas to form galvanic corrosion at those localized spots?  I'm still leaning towards pitting corrosion but I can't be certain.  Any more insight?
EdStainless (Materials)
8 Jan 09 8:43
One possibility is that the Al corrodes so easily that it forms a layer of aluminum oxide between the fin and steel.  The porous oxide residue then traps impurities which will attack the steel.  Since the local portion of the Al is already corroded it can provide galvanic protection.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

IFRs (Petroleum)
8 Jan 09 9:15
Could there be condensation bridging the small gap between the fin and the pipe, setting up a galvanic cell?
swall (Materials)
8 Jan 09 9:32
It could be stray current corrosion, if the ground potential of the tubing is less than the ground potential of the aluminum.
a7992 (Industrial) (OP)
8 Jan 09 9:34
Yeah there definitely could be condensation on the outside...if it is Galvanic corrosion wouldn't Al be the one corroding since it is more active or do I have it backwards?  I think galvanic corrosion is the most plausible but are these two metals susceptible?
a7992 (Industrial) (OP)
8 Jan 09 9:39
On the sample I have, the holes are only on one side (the thinning side) and I'm pretty sure the fins are around the entire tube.  Would the thinning tube make that side only more susceptible to galvanic corrosion?

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